During the first trimester of my pregnancy, I had a lingering cough that would not go away. I thought it was allergies, so I procrastinated about going to the doctor. After all, I was a 33-year-old fitness instructor in top health.

But after I coughed so hard I fractured a rib, I saw several doctors and underwent both an X-ray and a CT scan. That’s how I learned of the 16-centimeter mass sitting on top of my left lung.

My...